An aircraft lifting surface (see FIG. 1a) is usually structured by leading edges 11, torsion boxes 13, trailing edges 15 with control surfaces (flaps, elevators, rudders, etc.), a root joint 17 and tips 18.
A torsion box 13 is a primary structure responsible of supporting all loads involved (aerodynamic, fuel, dynamics, etc.) and comprises several structural elements. In the known configuration shown in FIG. 1b said structural elements are upper and lower skins 31, 33 stiffened by longitudinal stringers, a front spar 19, a rear spar 20 and transversal ribs 37 attached to the frontal and rear spars 19, 20 and to the upper and lower skins 31, 33 in order to keep the torsion box shape and reinforce the load introductions areas linked to the structural arrangement to the rest of the structure and to the actuators for handling the control surfaces.
The leading and trailing edge edges 11, 15 are structures responsible for keeping the overall aerodynamic shape of the aircraft lifting surface.
A known leading edge 11 (see FIG. 1c) comprises, on the one hand, several ribs 21, called leading edge ribs, attached to the front spar 19 of the torsion box 13 and, on the other hand, an aerodynamic profile 25 attached to the leading edge ribs 21 and to the flanges of the front spar 19.
Similarly the trailing edge 15 comprises trailing edge ribs attached to the rear spar 20 and an aerodynamic profile attached to the trailing edge ribs and to the flanges of the rear spar 20.
Nowadays, and particularly in the aeronautical industry, composite materials with an organic matrix and continuous fibers, especially CFRP (Carbon Fiber Reinforced Plastic) are widely used in a great variety of structural elements. Specifically, all the elements which make up the aforementioned torsion boxes 13 can be manufactured using CFRP.
Typically, all structural elements forming a torsion box (upper and lower skins 31, 33, front and rear spar 19, 20 and ribs 37) are manufactured separately and then joined by means of rivets with the aid of complicated tooling to achieve the necessary tolerances, which are given by the aerodynamic, assembly and structural requirements.
A well-known method for manufacturing said elements uses prepreg technology. In a first step, a flat lay-up of composite prepreg plies for each element is prepared. Then a laminated preform of the element with the required shape is obtained by means of a classical hot-forming process, being in some cases substituted by a press-forming process due to high curvatures. After getting the required shape, the laminated preform is cured in a male or female tooling depending on the tolerances required and the overall manufacturing cost. In the case of certain elements comprising sub-components cured separately, such as a rib 37 and a vertical stiffener of it, a second curing cycle is needed for co-bonding said sub-components. Finally, after all the curing cycles, the element contours are trimmed getting the final geometry, and then the element is inspected by an ultrasonic system to assure its quality.
The cost of the inner structure of a torsion box manufactured with said method is high because said steps shall be carried out independently for each element (spars 19, 20 and ribs 37).
Additionally, the cost related to the assembly of the torsion box is also high due to the long length and high complexity of the tasks required to install and to fit all inner elements like spars 19, 20 and ribs 37 together. First of all, the front and rear spars 19, 20 are located in the assembly jig, ensuring the interfaces with the leading and trailing edges 11, 15. Then, all the ribs 37 are located using the vertical stiffeners located on both spars 19, 20 as references and are joined to them by means of bolts. This rib assembly is as long as the number of ribs demands. After assembling the inner structure of the torsion box both upper and lower skins 31, 33 are located over the inner structure, joining them to spars 19, 20 and ribs 37 with rivets, in a final stage closing the whole torsion box.
After the assembly of the torsion box and prior to the installation of the leading and trailing edge aerodynamic profiles, the leading and trailing edge ribs are positioned and bolted to, respectively, the front and rear spars 19, 20 using outer stiffeners located on them as references. The assembly of these leading and trailing edge ribs involves additional costs to the ones related to the main torsion box assembly process.